Butanol (CH3CH2CH2CH2OH), a four-carbon alcohol, can be used as a liquid fuel to run engines such as cars. Butanol can replace gasoline and the energy contents of the two fuels are nearly the same (110,000 Btu per gallon for butanol; 115,000 Btu per gallon for gasoline). Butanol has many superior properties as an alternative fuel when compared to ethanol as well. These include: 1) Butanol has higher energy content (110,000 Btu per gallon butanol) than ethanol (84,000 Btu per gallon ethanol); 2) Butanol is six times less “evaporative” than ethanol and 13.5 times less evaporative than gasoline, making it safer to use as an oxygenate and thereby eliminating the need for very special blends during the summer and winter seasons; 3) Butanol can be transported through the existing fuel infrastructure including the gasoline pipelines whereas ethanol must be shipped via rail, barge or truck; and 4) Butanol can be used as replacement for gasoline gallon for gallon e.g. 100% or any other percentage, whereas ethanol can only be used as an additive to gasoline up to about 85% (E-85) and then only after significant modification to the engine (while butanol can work as a 100% replacement fuel without having to modify the current car engine).
A significant potential market for butanol as a liquid fuel already exists in the current transportation and energy systems. Butanol is also used as an industrial solvent. In the United States, currently, butanol is manufactured primarily from petroleum. Historically (1900s-1950s), biobutanol was manufactured from corn and molasses in a fermentation process that also produced acetone and ethanol and was known as an ABE (acetone, butanol, ethanol) fermentation typically with certain butanol-producing bacteria such as Clostridium acetobutylicum and Clostridium beijerinckii. When the USA lost its low-cost sugar supply from Cuba around 1954, however, butanol production by fermentation declined mainly because the price of petroleum dropped below that of sugar. Recently, there is renewed R&D interest in producing butanol and/or ethanol from biomass such as corn starch using Clostridia- and/or yeast-fermentation process. However, similarly to the situation of “cornstarch ethanol production,” the “cornstarch butanol production” process also requires a number of energy-consuming steps including agricultural corn-crop cultivation, corn-grain harvesting, corn-grain starch processing, and starch-to-sugar-to-butanol fermentation. The “cornstarch butanol production” process could also probably cost nearly as much energy as the energy value of its product butanol. This is not surprising, understandably because the cornstarch that the current technology can use represents only a small fraction of the corn crop biomass that includes the corn stalks, leaves and roots. The cornstovers are commonly discarded in the agricultural fields where they slowly decompose back to CO2, because they represent largely lignocellulosic biomass materials that the current biorefinery industry cannot efficiently use for ethanol or butanol production. There are research efforts in trying to make ethanol or butanol from lignocellulosic plant biomass materials—a concept called “cellulosic ethanol” or “cellulosic butanol”. However, plant biomass has evolved effective mechanisms for resisting assault on its cell-wall structural sugars from the microbial and animal kingdoms. This property underlies a natural recalcitrance, creating roadblocks to the cost-effective transformation of lignocellulosic biomass to fermentable sugars. Therefore, one of its problems known as the “lignocellulosic recalcitrance” represents a formidable technical barrier to the cost-effective conversion of plant biomass to fermentable sugars. That is, because of the recalcitrance problem, lignocellulosic biomasses (such as comstover, switchgrass, and woody plant materials) could not be readily converted to fermentable sugars to make ethanol or butanol without certain pretreatment, which is often associated with high processing cost. Despite more than 50 years of R&D efforts in lignocellulosic biomass pretreatment and fermentative butanol-production processing, the problem of recalcitrant lignocellulosics still remains as a formidable technical barrier that has not yet been eliminated so far. Furthermore, the steps of lignocellulosic biomass cultivation, harvesting, pretreatment processing, and cellulose-to-sugar-to-butanol fermentation all cost energy. Therefore, any new technology that could bypass these bottleneck problems of the biomass technology would be useful.
Oxyphotobacteria (also known as blue-green algae including cyanobacteria and oxychlorobacteria) and algae (such as Chlamydomonas reinhardtii, Platymonas subcordiformis, Chlorella fusca, Dunaliella salina, Ankistrodesmus braunii, and Scenedesmus obliquus), which can perform photosynthetic assimilation of CO2 with O2 evolution from water in a liquid culture medium with a maximal theoretical solar-to-biomass energy conversion of about 10%, have tremendous potential to be a clean and renewable energy resource. However, the wild-type oxygenic photosynthetic green plants, such as blue-green algae and eukaryotic algae, do not possess the ability to produce butanol directly from CO2 and H2O. The wild-type photosynthesis uses the reducing power (NADPH) and energy (ATP) from the photosynthetic water splitting and proton gradient-coupled electron transport process through the algal thylakoid membrane system to reduce CO2 into carbohydrates (CH2O)n such as starch with a series of enzymes collectively called the “Calvin cycle” at the stroma region in an algal or green-plant chloroplast. The net result of the wild-type photosynthetic process is the conversion of CO2 and H2O into carbohydrates (CH2O)n and O2 using sunlight energy according to the following process reaction:nCO2+nH2O→(CH2O)n+nO2  [1]The carbohydrates (CH2O)n are then further converted to all kinds of complicated cellular (biomass) materials including proteins, lipids, and cellulose and other cell-wall materials during cell metabolism and growth.
In certain alga such as Chlamydomonas reinhardtii, some of the organic reserves such as starch could be slowly metabolized to ethanol (but not to butanol) through a secondary fermentative metabolic pathway. The algal fermentative metabolic pathway is similar to the yeast-fermentation process, by which starch is breakdown to smaller sugars such as glucose that is, in turn, transformed into pyruvate by a glycolysis process. Pyruvate may then be converted to formate, acetate, and ethanol by a number of additional metabolic steps (Gfeller and Gibbs (1984) “Fermentative metabolism of Chlamydomonas reinhardtii,” Plant Physiol. 75:212-218). The efficiency of this secondary metabolic process is quite limited, probably because it could use only a small fraction of the limited organic reserve such as starch in an algal cell. Furthermore, the native algal secondary metabolic process could not produce any butanol. As mentioned above, butanol has many superior physical properties to serve as a replacement for gasoline as a fuel. Therefore, a new photobiological butanol-producing mechanism with a high solar-to-butanol energy efficiency is needed.
The present invention provides revolutionary designer photosynthetic organisms, which are capable of directly synthesizing butanol from CO2 and H2O using sunlight. The photobiological butanol-production system provided by the present invention could bypass all the bottleneck problems of the biomass technology mentioned above.